ok, now it makes more sense 😉
i’d favour going to 2G when the 3G coverage starts to be too low. You don’t want to extend your 3G coverage beyond its nominal cell range. Indeed, that would generate more noise for every other 3G user.
So I would activate the 3G-2G reselection in PTM mode, just before it is too late.
I guess you already actiavted this reselection in Idle Mode ?
And of course, activate it the other way around, but with enough hysteresis to avoid ping pong reselections…
(I know it all sounds very theoretical, but finding the perfect solution for you would require drivetests and extensive stats analysis…)